As high-speed transistors for communications, HEMTs (High Electron Mobility Transistors) are known that include the first generation GaAs HEMTs, the second generation InP HEMTs, followed by Sb HEMTs. In a Sb HEMT, InAs or InSb that has extremely light effective mass of an electron in the Γ valley is used for a channel semiconductor to form an electron transit layer. Here, the effective mass of an electron in the Γ valley is 0.067 me for GaAs, 0.043 me for In0.53Ga0.47As, 0.036 me for In0.7Ga0.3As, 0.031 me for In0.8Ga0.2As, 0.022 me for InAs, and 0.014 me for InSb. Here, me is the electron rest mass. By using InAs or InSb for an electron transit layer, the velocity of electrons can be made higher, hence there is a possibility to obtain a HEMT that can operate in a terahertz band.
If using InAs for an electron transit layer, it is often the case that AlSb, AlGaSb or AlAsSb is used for a barrier layer to confine electrons. Also, if using InSb for an electron transit layer, it is often the case that InAlSb is used for a barrier layer to confine electrons. Among these cases, if using InAs with the lattice constant of 0.6058 nm for an electron transit layer, it is comparatively preferable when forming a heterostructure because InAs, AlSb, and GaSb have substantially the same lattice constant of about 0.61 nm.
[Patent Document]    Japanese Laid-open Patent Publication No. 2007-81103
FIG. 1 illustrates a cross-sectional structure of an AlSb/InAs HEMT whose electron transit layer is formed with InAs, and the barrier layer is formed with AlSb. FIG. 2 is a band structure diagram of the HEMT whose structure is illustrated in FIG. 1.
The HEMT with this structure has a semi-insulating GaAs substrate 910, above which graded layers are formed with a buffer layer 911, an i-AlSb barrier layer 912, an i-InAs channel layer 913, an i-AlSb spacer layer 914, a Te-δ doping area 915, an i-AlSb electron barrier layer 916, an i-InAlAs hole barrier layer 917, and an n-InAs cap layer 918. On the n-InAs cap layer 918, a source electrode 931 and a drain electrode 932 are formed, and a gate electrode 933 is formed on the i-InAlAs hole barrier layer 917. Also, except for the area where the gate electrode 933 is formed, the area between the source electrode 931 and the drain electrode 932 on the n-InAs cap layer 918 or the like is covered with a silicon dioxide film 920. Here, in the HEMT with this structure, the i-InAs layer 913 is an electron transit layer.
The AlSb/InAs heterostructure in the HEMT has the so-called “type-II band structure”, which is different from a GaAs or InP HEMT in that electrons and holes are spatially separated. Therefore, electrons exist in the i-InAs channel layer 913, and holes that are generated by impact ionization due to small bandgap energy exist in the i-AlSb electron barrier layer 916. Here, two dimensional electron gas (2DEG) 913a is formed in the i-InAs channel layer 913.
In the HEMT with this structure, a leakage current to the gate electrode 933 is suppressed because the i-AlSb spacer layer 914 and the i-AlSb electron barrier layer 916 are barrier layers of electrons. However, the i-InAlAs hole barrier layer 917 is required as a barrier layer for holes because holes exist in i-AlSb electron barrier layer 916. To make the i-InAlAs hole barrier layer 917 work sufficiently as a barrier layer for holes, it is required to be formed with a composition of InxAl1-xAs where x is set to around 0.4 to 0.5. Therefore, the i-InAlAs hole barrier layer 917 is put under a great tensile strain. In addition, it cannot be formed with much thickness, and crystal quality is not so good, which makes it difficult to suppress a leak of holes to the gate electrode 933 completely.
Thus, AlSb/InAs HEMT has a type-II heterostructure that needs to have not only a barrier for electrons but also a barrier for holes to suppress a leakage current to the gate electrode 933. Namely, it is required that both the i-AlSb electron barrier layer 916 or the like and the i-InAlAs hole barrier layer 917 are formed. This makes the total thickness of the barrier layers greater, which in turn makes it difficult to suppress a short-channel effect when refining the gate length Lg. Here, to suppress the short-channel effect, a channel aspect ratio Lg/d needs to be kept high as much as possible. Here, d denotes the distance between a gate and a channel.
As a method to suppress a leak of holes to the gate electrode 933 without forming the i-InAlAs hole barrier layer 917, as illustrated in FIG. 3, a method is known in which a Te-δ doping area 915 is formed close to the side where the gate electrode 933 is disposed. FIG. 4 is a band structure diagram of the HEMT that has the structure illustrated in FIG. 3. As illustrated, the band in the i-AlSb spacer layer 914 is bent to work as a barrier for holes. However, in the HEMT with this structure, the position where Te-δ doping area 915 is formed is away from the i-InAs channel layer 913, which makes the density of electrons in the i-InAs channel layer 913 low. Namely, the density of the 2DEG 913a is reduced, resulting in lowered electrical characteristics in the HEMT. Here, in FIG. 3, the structure has an i-InAlAs hole barrier layer 917 formed. In the HEMT with this structure, however, a barrier for holes may be formed without forming the i-InAlAs hole barrier layer 917. Therefore, in the HEMT with this structure, an i-InAlAs hole barrier layer 917 does not need to be formed necessarily, hence it is possible to make the total thickness of barrier layers thinner.
Also, as illustrated in FIG. 5, there is a method of forming a Te-δ doping area 915 and a Te-δ doping area 919 in an i-AlSb spacer layer 914 and an i-AlSb electron barrier layer 916. FIG. 6 is a band structure diagram of the HEMT that has the structure illustrated in FIG. 5. By forming two Te-δ doping areas, it is possible to make the density of electrons higher in the i-InAs channel layer 913. However, the HEMT with this structure cannot confine holes sufficiently for the same reason as with the structure illustrated in FIG. 1.